1. Field of the Invention
The present invention relates to an active matrix type display device including an organic EL (Electroluminescence) light emitting element or the like.
2. Description of the Related Art
In an image display device, for example, a liquid crystal display or the like, a large number of pixels are arranged in the form of a matrix, and light intensity is controlled for each pixel according to information on an image to be displayed, whereby the image is displayed.
While the same is true for an organic EL display or the like, the organic EL display is a so-called emissive display having a light emitting element in each pixel circuit, and it has advantages of providing a higher image viewability, eliminating a need for a backlight, and a having higher response speed, for example, as compared with the liquid crystal display.
In addition, the organic EL display differs greatly from the liquid crystal display or the like in that the luminance of each light emitting element is controlled by the value of a current flowing through the light emitting element, and thereby color gradation is obtained, that is, in that the light emitting element is of a current-controlled type.
As in the liquid crystal display, there are a simple matrix system and an active matrix system as possible driving systems of the organic EL display. The former has a simple structure, but presents problems including, for example, difficulty in realizing a large high-definition display. Therefore, the active matrix system, which controls a current flowing through a light emitting element within each pixel circuit by an active element, or typically a TFT (Thin Film Transistor), provided within the pixel circuit, has been actively developed.
FIG. 1 is a block diagram showing a configuration of an ordinary organic EL display device.
As shown in FIG. 1, this display device 1 includes: a pixel array unit 2 having pixel circuits (PXLC) 2a arranged in the form of a m×n matrix; a horizontal selector (HSEL) 3; a write scanner (WSCN) 4; signal lines (data lines) SGL1 to SGLn selected by the horizontal selector 3 and supplied with a data signal corresponding to luminance information; and scanning lines WSL1 to WSLm selected and driven by the write scanner 4.
Incidentally, the horizontal selector 3 and the write scanner 4 may be formed on polycrystalline silicon, or by a MOSIC or the like on the periphery of the pixels.
FIG. 2 is a circuit diagram showing an example of a configuration of a pixel circuit 2a in FIG. 1 (see U.S. Pat. No. 5,684,365 (Patent Document 1) and Japanese Patent Laid-Open No. Hei 8-234683 (Patent Document 2)).
The pixel circuit of FIG. 2 has the simplest circuit configuration among a large number of circuits that have been proposed, and it is a circuit of a so-called two-transistor driving system.
The pixel circuit 2a of FIG. 2 includes a p-channel thin film field effect transistor (hereinafter referred to as a TFT) 11 and a TFT 12, a capacitor C11, and an organic EL light emitting element (OLED) 13 as a light emitting element. In FIG. 2, SGL denotes a signal line, and WSL denotes a scanning line.
The organic EL light emitting element has a current rectifying property in many cases, and therefore may be referred to as an OLED (Organic Light Emitting Diode). Although the symbol of a diode is used for a light emitting element in FIG. 2 and other figures, the current rectifying property is not necessarily required of an OLED in the following description.
In FIG. 2, the source of the TFT 11 is connected to a power supply potential VCC. The cathode of the light emitting element 13 is connected to a ground potential GND. The operation of the pixel circuit 2a of FIG. 2 is as follows.
Step ST1:
When the scanning line WSL is set in a selected state (a low level in this case), and a writing potential Vdata is applied to the signal line SGL, the TFT 12 conducts to charge or discharge the capacitor C11, and the gate potential of the TFT 11 becomes the writing potential Vdata.
Step ST2:
When the scanning line WSL is set to a non-selected state (a high level in this case), the signal line SGL and the TFT 11 are electrically disconnected from each other. However, the gate potential of the TFT 11 is maintained stably by the capacitor C11.
Step ST3:
A current flowing through the TFT 11 and the light emitting element 13 has a value corresponding to the gate-to-source voltage Vgs of the TFT 11, and the light emitting element 13 continues emitting light at a luminance corresponding to the current value.
An operation of selecting the scanning line WSL and transmitting the luminance information supplied to the data line to the inside of the pixel as in the above-described step ST1 hereinafter will be referred to as “writing”.
As described above, once the writing potential Vdata is written in the pixel circuit 2a of FIG. 2, the light emitting element 13 continues emitting light at a constant luminance until the writing potential Vdata is next rewritten.
As described above, in the pixel circuit 2a, the value of the current flowing through the EL light emitting element 13 is controlled by changing the voltage applied to the gate of the TFT 11 as a driving (drive) transistor.
At this time, the source of the p-channel drive transistor is connected to the power supply potential VCC, and the TFT 11 operates in a saturation region at all times. The TFT 11 is therefore a constant-current source having a value expressed by the following Equation 1.(Equation 1)Ids=½·μ(W/L)Cox(Vgs−Vth|)2  (1)where μ denotes carrier mobility, Cox denotes gate capacitance per unit area, W denotes gate width, L denotes gate length, Vgs denotes the gate-to-source voltage of the TFT 11, and Vth denotes the threshold value of the TFT 11.
Each light emitting element in a simple matrix type image display device emits light only at a moment when the light emitting element is selected. On the other hand, the light emitting element in the active matrix system continues emitting light even after writing is ended, as described above. The active matrix system is therefore advantageous, especially in a large high-definition display, in that the peak luminance and the peak current of the light emitting element can be decreased as compared with the simple matrix system, for example.
FIG. 3 is a diagram showing a secular change in the current-voltage (I-V) characteristic of the organic EL light emitting element. In FIG. 3, a curve represented by a solid line indicates a characteristic at a time of an initial state, and a curve represented by a broken line indicates a characteristic after a secular change.
Generally, as shown in FIG. 3, the I-V characteristic of the organic EL light emitting element is degraded with the passage of time.
However, because the two-transistor driving of FIG. 2 is constant-current driving, a constant current continues flowing through the organic EL light emitting element as described above, and the light emission luminance of the organic EL light emitting element is not degraded with time even when the I-V characteristic of the organic EL light emitting element is degraded.
The pixel circuit 2a of FIG. 2 is formed with p-channel TFTs. When the pixel circuit 2a of FIG. 2 can be formed with n-channel TFTs, an existing amorphous silicon (a-Si) process can be used in TFT fabrication. Thereby, the cost of a TFT substrate can be reduced.
Next, a description will be made of a basic pixel circuit in which the transistors are replaced with n-channel TFTs.
FIG. 4 is a circuit diagram showing a pixel circuit in which the p-channel TFTs in the circuit of FIG. 2 are replaced with n-channel TFTs.
The pixel circuit 2b of FIG. 4 includes n-channel TFTs 21 and 22, a capacitor C21, and an organic EL light emitting element (OLED) 23 as a light emitting element. In FIG. 4, SGL denotes a data line, and WSL denotes a scanning line.
In this pixel circuit 2b, the drain side of the TFT 21 as drive transistor is connected to a power supply potential VCC, and the source of the TFT 21 is connected to the anode of the EL light emitting element 23, whereby a source follower circuit is formed.
FIG. 5 is a diagram showing an operating point of the TFT 21 as a drive transistor and the EL light emitting element 23 in an initial state. In FIG. 5, an axis of abscissas indicates the drain-to-source voltage Vds of the TFT 21, and an axis of ordinates indicates the drain-to-source current Ids of the TFT 21.
As shown in FIG. 5, source voltage is determined by the operating point of the TFT 21 as a drive transistor and the EL light emitting element 23, and the voltage has a different value depending on the gate voltage.
Because the TFT 21 is driven in a saturation region, the TFT 21 passes the current Ids having the current value of the equation shown as the above Equation 1 relating to the gate-to-source voltage Vgs corresponding to the source voltage at the operating point.
The active matrix type organic EL display of the constitution as described above has a laminated structure of a TFT circuit for driving the EL light emitting element and the EL light emitting element as a light emitting layer. Active matrix type organic EL displays of a top emission structure and a bottom emission structure are generally known.
For example, a passive matrix type organic EL element of a top emission structure is disclosed in Japanese Patent Laid-Open No. 2005-203196 (Patent Document 3).
In the case of an ordinary bottom emission structure, a light emission needs to be extracted from a TFT substrate side, and therefore a TFT structure may impose a limitation of EL light emission area=aperture ratio.
In the case of a top emission structure, on the other hand, a light emission needs to be extracted from a counter substrate side. Therefore, the top emission structure is not affected by the TFT structure, so that the EL light emission area=aperture ratio can be increased.